Very nice. Would you mind showing us the node graph? Not really sure how to correctly setup the nodegroup to get such leather.

As they say:"For the leather material on the chair model, we used a thin film of index η2 = 1.3 and thickness d = 290nm,
over a rough dielectric base material (η3 = 1). When the scene is rotated, goniochromatic effects such as subtle
purple colors may be observed at grazing angles."

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Right, I don't remember what exact setup was but tried again. The setup was very simple.
Sorry had to remove wooden chair parts because of the forum upload limits.

I didn't play much with that leather but you may get better results I believe

Too smooth, but I'm a bit confused looking at your example.
A minimum value lower than 50, it is so thin that film reflects no light and is often called the black film.
Against the dark background, these areas will be virtually invisible as I understand.
That is kind of normal with bubble, but not in this case with leather I think.
Why you set the minimum value to zero?

If both min and max are set to 290 i didn´t got any effect with using a texture as thickness input. That´s why I set it to 0.

I didn´t thought about what the nodes physically do more how they behave in relation to each other.

It seems that the thickness color input maps the range between minimum and maximum film thickness.

Setting the minimum to 0 enables the thickness color input to represent the full range from 0 - 290 nm so I was able to input a texture. The gradient that recolored the texture started with a grey into white so I probably used a range between 100 nm - 290 nm (similar to your last example actually).

So I basically used the gradient to set the ranges and to see how they react visually.